Horizontal Gene Transfer of Staphylococcus aureus Biofilms
Biofilms Biofilms are major virulence factors when it comes to infection of certain types of microbes. They are thick matrices of polysaccharide material that proved adhesion to host sites as well as protection. The "protective" coat is known as an EPS, or Extracellular Polymeric Substance, and is a structure that allows efficient exchange of genetic material. Along with the EPS, Polysaccharide Intercellular Adhesins (PIA's) are sulfated polysaccharides that enhance the strength of biofilms and their formation. Biofilms however, at least in the case of Staphylococcus aureus, use Quorum Sensing to control the formation of the biofilm. When there are enough bacterial cells in the specific area, the agr Operon is able to become activated through extracellular signals, thus creating proteins needed for biofilm formation. By creating biofilms, Staphylococcus aureus has the ability to attach to surfaces, develop a microbial community, and acquire or disseminate plasmids of particular value. Moreover, these plasmids that are transferred from one cell to another may carry antibiotic resistance genes. Horizontal Gene transfer in Staphylococcus aureus Biofilm formation, as previously stated, is a method of protection used by the bacterial cells. It allows these bacterial communities to resist environmental strains such as antibiotic targeting and and even immune system targeting. Microbiologists are able to show that the bacterial cells within a biofilm have a better ability to mutate. These mutations that are invoked can very easily be heritable antibiotic resistant genes. Before the study named "Staphylococcus aureus Biofilms Promote Horizontal Gene Transfer of Antibiotic Resistance" was published, it was known that Staphylococcus aureus is able to efficiently undergo conjugal transfer when it is adhered to a surface. With this knowledge, microbiologists inferred that there would be efficient horizontal plasmid transfer that would promote biofilm formation. It is well understood that Staphylococcus aureus' Mobile Genetic Elements (MGE's) such as plasmids, transposons, or pathogenicity islands undergo Horizontal Gene transfer (HGT) when under selective pressures by the host. Studies using the prototypical multidrug resistance plasmid pGO1 and a mobilizable plasmid PC223 have proven to have a higher rate of gene transfer when a biofilm is present. The UAMS-1 strain was also used in order to backup the data recovered. Horizontal Gene Transfer Study The Staphylococcus aureus strains used were SH1000 and SH1000∆sigB (unable to form a biofilm). The study performed verified previous studies in the fact that planktonic growth (more singular existing cells) is not favorable for conjugation and mobilization of the specific plasmids. Using Mueller Hinton Agar, which is used to determine antibiotic susceptibility, the frequencies of conjugation and mobilization were measured. Conjugation frequencies were based on the amount of pGO1 plasmids given up per donor. Mobilization frequency described as the amount of pC233 per donor and the mobilization efficiency was based on the amount of transconjugants with both pGO1 AND pC223. Static broth culture showed no evidence of gene transfer for SH1000 strains, which is contrasted by "optimal standard filter mating conditions" and biofilm cultures. This can be seen in Figure 1. Figure one provides a visual on transfer and conjugation frequencies. They found that the recipient strain SH1000-R was Novobiocin and Rifampin resistant (up to 5µg/mL) once receiving pGO1.The study further concluded that simultaneous transfer of pC223 and pGO1 were most effective under biofilm growth in comparison to planktonic or standard filter mating conditions. Using the Staphylococcus aureus strain UAMS-1 concluded that the study results were not strain specific, conjugation frequencies for biofilms was proven to be 16,000-fold higher than planktonic frequencies. This is all conclusive evidence that recipient strains of the transferred genetic material is predominantly due to the biofilm formation of Staphylococcus aureus. References http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3623343/ http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2732559/ http://en.wikipedia.org/wiki/Bacterial_conjugation